This invention relates generally to the field of data communications and, more particularly, to a circuit for the convenient connection together of multiple serial data lines.
The technology of data communications is primarily concerned with moving data from one discrete device to another, most commonly via cable. The point of connection between a device and the attached cable is called an interface. Unfortunately, the physical characteristics of data communications cable as well as the physical and electrical characteristics of the interface connections of different devices are far from uniform, which has given rise to the requirement for a standard. In the last several years, the number of interfaces has narrowed into a small group of widely used standards, the Electronic Industries Association Standard RS-232-C currently being the most universal. The standard defines the electrical characteristics for an interface between a piece of data terminal equipment (DTE) and a piece of data communications equipment (DCE). The DTE is typically a user's terminal but may also be a computer. The DCE is typically a modulator-demodulator (modem) which encodes the computer data, e.g., for telephone line transmission.
FIG. 1 illustrates the cable plug and the wires associated with the RS-232-C standard. The connector has twenty-five (25) pins, not all of which are used, however, in most applications. Pins 2 and 3 are the data-carrying wires, called transmitted and received data, respectively. Pin 7 is a signal ground which serves as a signal current return path. These three wires are sufficient for communication of data between DCE and DTE, and selected others of the wires serve as control wires between the DTE and DCE for establishing and maintaining communications with the computer, that is, for providing the handshaking function; pins 4, 5, 6, 8 and 20 are normally used for this purpose. Pin 1 is a protection ground. The pins not specifically called out above are not involved in the operation of the present invention and will, therefore, be ignored in the description to follow.
The EIA RS-232-C standard also includes the Interchange Equivalent Circuit, reproduced as FIG. 2, which shows the electrical parameters which are specified in the standard. The equivalent circuit is applicable to all interchange circuits regardless of the category (data, timing, or control) to which they belong, and is independent of whether the driver is located in the DCE and the terminator in the DTE or vice versa. The electrical parameters shown in FIG. 2 are as follows:
V.sub.o is the open-circuit driver voltage. PA1 R.sub.o is the driver internal dc resistance. PA1 C.sub.o is total effective capacitance associated with the driver, measured at the interface point and including any cable to the interface point. PA1 V.sub.1 is the voltage at the interface point. PA1 C.sub.L is the total effective capacitance associated with the terminator, measured at the interface point and including any cable to the interface point. PA1 R.sub.L is the terminator load dc resistance. PA1 E.sub.L is the open-circuit terminator voltage (bias).
For data interchange circuits, the signal is in the marking condition (i.e., a logic "1") when the voltage V.sub.1 is more negative than -3 volts with respect to signal ground, and is in the spacing condition (i.e., a logic "0") when the voltage V.sub.1 is more positive than +3 volts with respect to signal ground. The region between +3 volts and -3 volts is a transition region in which the signal state is not uniquely defined. For timing and control interchange circuits, the function is considered ON when the voltage V.sub.1 is more positive than +3 volts with respect to circuit AB, and is considered OFF when the voltage V.sub.1 is more negative than -3 volts with respect to circuit AB. The function is not uniquely defined for voltages in the transition region between +3 volts and -3 volts.
The load impedance of the terminator side of the interface has a dc resistance (R.sub.L) of not less than 3,000 ohms, measured with an applied voltage not greater than 25 volts, nor more than 7,000 ohms, measured with an applied voltage of 3 to 25 volts. The effective shunt capacitance (C.sub.L) shall not exceed 2500 picofarads and the open circuit terminator voltage (E.sub.L) shall not exceed two volts.
The open-circuit driver voltage (V.sub.o) with respect to circuit AB on any interchange circuit shall not exceed 25 volts. The source impedance is not specified, but the combination of V.sub.o and R.sub.o shall be selected such that a short circuit between any two conductors in the interconnecting cable shall not result in a current in excess of one-half ampere. Additionally, the driver design shall be such that, when the terminator load resistance (R.sub.L) is in the range between 3,000 ohms and 7,000 ohms and the terminator open-circuit voltage (E.sub.L) is zero, the potential V.sub.1 at the interface point shall not be less than 5 volts nor more than 15 volts in magnitude.
Frequently it is desirable to be able to conveniently interconnect two or more terminals (DTE) to one port of a computer, or to connect one terminal and one printer to one computer port or modem so that the DTEs can access the computer in parallel. As another example, it is frequently desirable to interconnect a computer in common with a telephone modem and a terminal, the terminal to be used with the computer while the user is at the office, and the telephone modem enabling the user to access the computer from home and to proceed from exactly where he/she was when he/she left the office, without need for manual switching. Another situation in which it is advantageous for several terminals to have access to a common computer is when the vendor of the computer hardware and/or software wishes to do debugging via telephone from a remote location and the owner wishes to observe what is going on and to occasionally make his/her own inputs; this requires the interconnection with the computer of a telephone modem and a terminal. In order to achieve such interconnection with a minimum of inconvenience it is desirable that the function be achieved without requiring an external source of power.
A known device for accomplishing the described function is the "Modem Splitter" commercially available from BLACK BOX Corporaton of Pittsburgh, Pa., which is a passive device requiring no external power source. The device, which is housed in a box-like enclosure, has four connectors conforming to the RS-232-C interface standard mounted thereon, one of which is adapted for attachment to a modem, with the others adapted for attachment to and interconnection with up to three terminals or monitors. As shown in FIG. 3, wherein only two connectors 10 and 12 (for connection to respective data terminals) and a connector 14 for plug-in application to a modem, are shown, the device utilizes diode circuits 16 and 18, each consisting of a diode and a 10K ohms resistor connected in parallel therewith, connected from the transmitted data pin 2 of each of the DTE connectors to pin 2 of the DCE connector. The received data lines (pin 3) and the signal ground lines (pin 7) are connected together as shown; lines 4 and 20, which are utilized to establish and maintain communication between the terminals and the modem, as well as such other line as might be involved in providing the handshaking function, have been omitted for clarity.
While this product is relatively inexpensive and performs acceptably in a variety of applications, applicants have found that under certain conditions the device fails to operate satisfactorily, or does not conform to the R-232-C standard, particularly at bit rates above 9600 bits/second. The equivalent circuits shown in FIG. 4A and FIG. 4B of two possible signal/circuit conditions are illustrative of situations in which the BLACK BOX device fails to conform to the standard. FIG. 4A is the equivalent circuit of two DTEs 10' and 12', each having zero internal dc reistance (R.sub.o) interconnected by the BLACK BOX device to a DCE 14' having a dc resistance R.sub.L of 4K ohms, and with the input from DTE 10' being plus and the input from DTE 12' being minus. The plus input signal from DTE 10' forward-biases diode 16 (FIG. 3) into conduction and is approximated as a short circuit. The minus input signal for DTE 12' reverse-biases its associated diode, thereby putting the 10K resistor of network 18 in parallel with R.sub.L, which has been assumed to have a value of 4K ohms (within the RS-232-C standard). Thus, the equivalent load to either DTE is (4.times.10).div.(4+10)=2.8K ohms, which does not conform to the RS-232-C standard which, it will be recalled, specifies a terminator load resistance in the range between 3K ohms and 7K ohms.
FIG. 4B is the equivalent circuit of two DTEs 10' and 12', each having zero dc internal resistance (R.sub.o), interconnected by the BLACK BOX device to a DCE 14' having a dc resistance R.sub.L in the range of 3K to 7K ohms and a total effective capacitance of 2500 picofarads, with the input from one of the DTEs having just undergone a transition from plus to minus, the other remaining minus throughout. Just prior to the illustrated signal condition the capacitor would have been charged positively because the diode of one or the other of the circuits 16 or 18 would have been forward-biased; this capacitance must be rapidly discharged to enable the data signal to pass through the transition period within the time specified by the standard, which is not to exceed one millisecond or four percent of the the nominal duration of a signal element. Now, with both inputs minus both diodes are reverse-biased, and the two 10K resistors are put in parallel with each other thereby to put 5K ohms in parallel with the terminator load resistor; for a terminator resistance of 3K ohms the effective resistance is (3.times.5).div.8=1.875K ohms, giving a time constant of 4.69 microseconds, and for a terminator resistance of 7K ohms the time constant is 7.29 microseconds, both of which are sufficiently long as to affect the quality of the signal passing through the interchange, particularly at high bit rates.
Inasmuch as many terminal devices and computers typically have bit rates higher than this, the highest currently available being 19,200 bits/second, there is a need for a plug-in device for hooking up more than two EIA RS-232-C standard terminals, computers, or modems, which will operate reliably at bit rates higher than 9600 bits per second, yet having the feature of not requiring an external power source.